Hyperthyroidism means an overactive thyroid gland.
The opposite of hyper is hypo- (hyp- before a vowel or h) which means "under, deficient or below normal." Hypothyroidism for example refers to an underactive thyroid gland. A hypodermic needle is used to inject drugs under the skin. Hyphidrosis [" + hidro- , sweat] refers to a condition characterized by an abnormally decreased ability to secrete sweat.
The prefix syn- (sym- before b, p, and m) means "together, with or joined." Many herbal combinations make use of herbs that are synergistic [syn-, together + -erg, to work]. Two substances are said to be synergistic when they work together to produce an effect that is greater than the sum of their individual effects.
Two other words that use the prefix syn (or sym) are:
synapse - where two nerve cells connect or "come together"
sympathy [" + path, to feel] - to experience the suffering of another or to "feel together."
A related word to sympathy is empathy [em-, in, within + path, to feel or suffer] which means "to be aware of and understand the feelings of another" (able to get "inside" someone else, experiencing what they are feeling.)
Suffixes are added to the end of combining forms to change the meaning of the word in some way. We have already discussed several suffixes including -itis which means an inflammed condition or an inflammed part of the body. The combining form hepat- means having to do with the liver. Therefore, hepatitis means:
pain in the liver
inflammation of the liver
pain in a joint
inflammation of a joint